The present invention relates to a high strength, tough medium carbon alloy steel.
High strength structural steels are used extensively for components such as aircraft landing gear, missiles, rocket casings, armor plate and other defense applications. In addition, where such steels have high hardness and consequent abrasion resistance, they are used in mining operations (e.g., buckets for mining, comminution and other mineral processing operations). Also, the high strength steels can be substituted for other low strength steels for a saving in weight of structural components for use in bridges, buildings, ship building, automobile parts and the like. The limiting factor in the use of high strength steels is their toughness. In practice, toughness and ductility are required to resist crack propagation and ensure sufficient formability for successful fabrication of the steel into engineering components. Thus, there is a need for a high strength tough steel. For the mining industry, it would be a significant advantage to impart a high degree of hardness (e.g., R.sub.c hardness value of greater than 40) to such steels for use in buckets, liners, balls, and the like.
One high strength steel available commercially is designated SAE 4340. It has acceptable yield strength and hardness. However, it is characterized by a room temperature Charpy-V-Notch impact toughness of on the order of 10 ft-lbs. This is an unacceptably low value to resist the propagation of cracks under impact loading conditions.
A high strength, tough alloy steel is disclosed in J. McMahon and G. Thomas, Proc. Third Intern. Conf. on the Strength of Metals and Alloys, Cambridge, Inst. Metals, London, 1973, 1, p. 180. The disclosed product is a ternary iron-chromium-carbon steel. It discloses a microstructure including thin sheets of highly deformed retained interlath austenite surrounding the martensitic crystal laths. At page 181, it is stated that upon tempering at 200.degree. C., the austenite was observed less frequently while upon tempering at 400.degree. C. none was seen. The authors concluded that such tempering caused the retained austenite to transform to ferrite, followed by precipitation of interlath carbides, accompanied by a drop in toughness. An iron/0.35 weight % carbon/4 weight % chromium alloy exhibited a Charpy-V-Notch value of 12-15 ft/lbs and a plane strain fracture toughness (K.sub.Ic) on the order of 70 KSI-in.sup.1/2.